Mixed-species Cultures Research Articles

Leaf Gas Exchange and Nutrient Use Efficiency Help Explain the Distribution of Two Neotropical Mangroves under Contrasting Flooding and Salinity

Rhizophora mangleandLaguncularia racemosacooccur along many intertidal floodplains in the Neotropics. Their patterns of dominance shift along various gradients, coincident with salinity, soil fertility, and tidal flooding. We used leaf gas exchange metrics to investigate the strategies of these two species in mixed culture to simulate competition under different salinity concentrations and hydroperiods. Semidiurnal tidal and permanent flooding hydroperiods at two constant salinity regimes (10 g L−1and 40 g L−1) were simulated over 10 months. Assimilation (A), stomatal conductance (gw), intercellular CO2concentration (Ci), instantaneous photosynthetic water use efficiency (PWUE), and photosynthetic nitrogen use efficiency (PNUE) were determined at the leaf level for both species over two time periods.Rhizophora manglehad significantly higher PWUE than didL. racemosaseedlings at low salinities; however,L. racemosahad higher PNUE andgwand, accordingly, had greater intercellular CO2(calculated) during measurements. Both species maintained similar capacities forAat 10 and 40 g L−1salinity and during both permanent and tidal hydroperiod treatments. Hydroperiod alone had no detectable effect on leaf gas exchange. However, PWUE increased and PNUE decreased for both species at 40 g L−1salinity compared to 10 g L−1. At 40 g L−1salinity, PNUE was higher forL. racemosathanR. manglewith tidal flooding. These treatments indicated that salinity influences gas exchange efficiency, might affect how gases are apportioned intercellularly, and accentuates different strategies for distributing leaf nitrogen to photosynthesis for these two species while growing competitively.

Substrate utilisation by Acidianus brierleyi, Metallosphaera hakonensis and Sulfolobus metallicus in mixed ferrous ion and tetrathionate growth media

When presented with ferrous ions and tetrathionate in growth media, individual archaeal species, Acidianus brierleyi, Metallosphaera hakonensis and Sulfolobus metallicus, exhibited different patterns of substrate utilisation and cell growth. Total cell numbers for A. brierleyi were about 60% of those for M. hakonensis and S. metallicus after complete substrate utilisation, and specific growth rates were affected by substrate concentration and cell growth history. High iron(II) concentrations inhibited S. metallicus growth. In relation to mixed microbial communities in high-temperature bioleaching reactors, the results are consistent with the hypothesis that specific growth rates of different species grown on different substrates strongly influence community structure. The combination of T-RFLP with substrate analysis and cell counts is a valuable analytical tool for the discrimination and semi-quantitative estimation of species in mixed cultures that can be used to gain better understanding of community structures and population dynamics in complex bioleaching systems.

A flow cytometric method for viability assessment of Staphylococcus aureus and Burkholderia cepacia in mixed culture

Mixed bacterial communities are commonly encountered in microbial infections of humans. Knowledge on the composition of species and viability of each species in these communities allows for a detailed description of the complexity of interspecies dynamics and contributes to the assessment of the severity of infections. Several assays exist for quantification of specific species in mixed communities, including analysis of quantitative terminal restriction fragment length polymorphisms. While this method allows for species-specific cell enumeration, it cannot provide viability data. In this study, flow cytometry was applied to assess the viability of Staphylococcus aureus and Burkholderia cepacia in mixed culture by membrane integrity analysis using SYBR® Green I and propidium iodide staining. Both bacteria are relevant to pulmonary infections of cystic fibrosis patients. Fluorescence staining was optimized separately for each species in pure culture due to differences between species in cell wall structure and metabolic capabilities. To determine viability of species in mixed culture, a protocol was established as a compromise between optimum conditions determined before for pure cultures. This protocol allowed the detection of viable and dead cells of both species, exhibiting an intact and a permeabilized membrane, respectively. To discriminate between S. aureus and B. cepacia, the protocol was combined with Gram-specific fluorescent staining using wheat germ agglutinin. The established three-color staining method was successfully tested for viability determination of S. aureus and B. cepacia in mixed culture cultivations. In addition, growth of both species was monitored by quantitative terminal restriction fragment length polymorphisms. The obtained data revealed alterations in viability during cultivations for different growth phases and suggest interspecies effects in mixed culture. Overall, this method allows for rapid simultaneous Gram-differentiation and viability assessment of bacterial mixed cultures and is therefore suitable for the analysis of dynamics of mixed communities of medical, environmental, and biotechnological relevance.

Suppression of bacteriocin production in mixed-species cultures of lactic acid bacteria

In the present work, the effect of co-culture on bacteriocin production by selected lactic acid bacteria (LAB) was studied. Thus, the co-culture of two bacteriocin-producer strains, Lactobacillus plantarum NC8C and Enterococcus faecium 6T1a, resulted in a significant reduction or even suppression of bacteriocin production by both strains. The phenomenon was not restricted to these two strains since we have further proved that it was more general and also occurred in several other co-cultures with LAB isolated from different food fermentations. This decrease in bacteriocin production was confirmed to be non cell–cell contact-dependent by using a two-compartment system in which the microorganisms were separated by a 0.2-μm-pore-size membrane. In addition, cell-free supernatants from certain co-cultures were able to suppress bacteriocin production in other strain cultures. In light of these findings, we hypothesize that such interactions between microorganisms should be taken into account to improve the development of bacteriocin-producing starter cultures used in food fermentations in order to avoid a negative effect on the quality of the fermented product.

Screening for competition effects and allelochemicals in benthic marine diatoms and cyanobacteria isolated from an intertidal flat (southern North Sea)

Scholz B. and Liebezeit G. 2012. Screening for competition effects and allelochemicals in benthic marine diatoms and cyanobacteria isolated from an intertidal flat (southern North Sea). Phycologia 51: 432–450. DOI: 10.2216/11-80.1Marine intertidal flats are colonized by diatom-dominated biofilms, whereas coccoid and filamentous cyanobacteria usually occur only at some stages during the growth season. Increasingly, cyanobacteria were most abundant during warmer seasons, outcompeting other taxa and we assumed direct competition between diatoms and cyanobacteria. Therefore, the most abundant species (25 diatoms and 10 cyanobacteria) were isolated from an intertidal flat in the Lower Saxonian Wadden Sea (southern North Sea) and the isolates were screened for allelochemical production (e.g. alkaloids, flavonoids) and allelopathic activities (e.g. algicidal, antimicrobial). Screenings were performed in single-species and mixed cultures of two, three and six species. Only polyunsaturated fatty acids, terpenes and sterols were found in single-species cultures, but aldehydes, flavonoids, alkaloids and phenols were detected in biomass extracts from mixed cultures. Chroococcus dimidiatus and Anabaena torulosa (Cyanophyceae) dominated other species in mixed cultures of cyanobacteria and diatoms; alkaloids were present in these mixed cultures. Cyanobacteria outgrew diatoms in cultures with two or three species, whereas the diatom Navicula phyllepta (Bacillariophyceae) outcompeted other diatoms and accumulated aldehydes. Six species (three diatoms and three cyanobacteria) were combined in the final test, which was conducted under different nitrogen regimes (800 µM and 250 µM). Navicula phyllepta outgrew the cyanobacteria and other diatoms under low-nitrogen conditions. Although on the basis of the current screening it was impossible to draw conclusions about the effective organisms in the mixed-species treatments, we highlighted both various bioactivities and potential bioactive chemicals in single-species and mixed cultures for a variety of benthic intertidal diatom and cyanobacterial strains.

Reduction of Nitrogen and Phosphorus Content in Piggery Water with Algal Culture

Animal waste causes environmental problems like eutrophication of ground and water. The objective of this study was to fix the total nitrogen (TN) and total phosphorus (TP) in swine waste into biomass by investigating the performance of sole and mixed algae species culture with fast growth algae Scenedesmus obliquus and wild algae Chlorella sp. Wastewater concentrations varied from 25 to 100% and initial pH varied from 6 to 9 were conducted. The results indicated that the initial wastewater concentration and pH are crucial to biomass growth and the removing rate of TN and TP. It was found out that the maximum biomass growth rate of 73 mg/l/d was achieved with mixed species culture in 75% piggery wastewater (with TN and TP of 54 and 12 mg/l) with pH 7, meanwhile, the removing rate of TN and TP reached 87.3% and 64.3% with 10 days culture, respectively.

Evaluation of Multispecies Plant-Growth-Promoting Consortia for the Growth Promotion of Jatropha curcas L.

Direct interactions that occur between members of different microbial types often result in the promotion of key processes that benefit plant growth and health. In the present study, four isolates, Brevibacillus brevis (MS1), Bacillus licheniformis (MS3), Micrococcus sp. (MS4), and Acinetobacter calcoaceticus (MS5), were used to develop multispecies consortia. They have the ability to produce IAA, solubilize inorganic P, and produce ACC deaminase and siderophore. The growth profile of MS1 was similar in monospecies and mixed-species cultures, but about a 24 % increase in mean growth rate was recorded for MS5. They enhanced the growth of Jatropha curcas in individual trials. Plant growth further improved maximally when the three were applied together. All four strains enhanced Jatropha growth in greenhouse and field experiments. Co-inoculation provides the largest and most consistent increases in shoot weight, root weight, total biomass, shoot and root length, total chlorophyll, shoot width, and grain yield.

Enhancement of Fermentative Hydrogen Production in an Extreme-Thermophilic (70°C) Mixed-Culture Environment by Repeated Batch Cultivation

Repeated batch cultivation was applied to enrich hydrogen fermentative microflora under extreme-thermophilic (70°C) environment. Initial inoculums received from a hydrogen producing reactor fed with organic fraction of household solid wastes. In total seven transfers was conducted and maximum hydrogen yield reached 296 ml H(2)/g (2.38 mol/mol) glucose and 252 ml H(2)/g (2.03 mol/mol) for 1 and 2 g/l glucose medium, respectively. It was found that hydrogen production was firstly decreased and got increased gradually from third generation. Acetate was found to be the main metabolic by-product in all batch cultivation. Furthermore, the diversity of bacterial community got decreased after repeated batch cultivation. It was proved that repeated batch cultivation was a good method to enhance the hydrogen production by enriching the mixed cultures of dominant species.

Evaluation of HiCrome differential agar for speciation of candida

Background: In recent years, non-albicans Candida (NAC) are being increasingly reported in many laboratories. There is a need to diagnose them early as some of the species are resistant to antifungals. We evaluated the utility of HiCrome agar for speciation of Candida in comparison to conventional methods. Materials and Methods: Ninety-six isolates of Candida were speciated by morphology on corn meal agar (CMA), sugar assimilation tests (SAT), and HiCrome agar. Sensitivity, specificity, and positive predictive and negative predictive values were obtained. Results: Sensitivity of HiCrome agar in identification of Candida albicans was 97.05%, C. tropicalis was 83.33%, C. glabrata 100%, and C. krusei 87.5%; and specificity was 98.07, 91.93, 92, and 92.30%, respectively. Conclusion: The present Candida differential agar is useful in identifying a few of the species. With further modifications this media may replace the need for conventional methods. The rapid availability of results and identification of species in mixed cultures is significant.

Differential response of Streptococcus mutans towards friend and foe in mixed-species cultures

In the oral biofilm, the 'mitis' streptococci are among the first group of organisms to colonize the tooth surface. Their proliferation is thought to be an important factor required for antagonizing the growth of cariogenic species such as Streptococcus mutans. In this study, we used a three-species mixed culture to demonstrate that another ubiquitous early colonizing species, Veillonella parvula, can greatly affect the outcome of the competition between a pair of antagonists such as S. mutans and Streptococcus gordonii. Transcriptome analysis further revealed that S. mutans responds differentially to its friend (V. parvula) and foe (S. gordonii). In the mixed culture with S. gordonii, all but one of the S. mutans sugar uptake and metabolic genes were downregulated, while genes for alternative energy source utilization and H₂O₂ tolerance were upregulated, resulting in a slower but persistent growth. In contrast, when cultured with V. parvula, S. mutans grew equally well or better than in monoculture and exhibited relatively few changes within its transcriptome. When V. parvula was introduced into the mixed culture of S. mutans and S. gordonii, it rescued the growth inhibition of S. mutans. In this three-species environment, S. mutans increased the expression of genes required for the uptake and metabolism of minor sugars, while genes required for oxidative stress tolerance were downregulated. We conclude that the major factors that affect the competition between S. mutans and S. gordonii are carbohydrate utilization and H₂O₂ resistance. The presence of V. parvula in the tri-species culture mitigates these two major factors and allows S. mutans to proliferate, despite the presence of S. gordonii.

Application of flow cytometry for the identification of Staphylococcus epidermidis by peptide nucleic acid fluorescence in situ hybridization (PNA FISH) in blood samples

Staphylococcus epidermidis is considered to be one of the most common causes of nosocomial bloodstream infections, particularly in immune-compromised individuals. Here, we report the development and application of a novel peptide nucleic acid probe for the specific detection of S. epidermidis by fluorescence in situ hybridization. The theoretical estimates of probe matching specificity and sensitivity were 89 and 87%, respectively. More importantly, the probe was shown not to hybridize with closely related species such as Staphylococcus aureus. The method was subsequently successfully adapted for the detection of S. epidermidis in mixed-species blood cultures both by microscopy and flow cytometry.

The development of Myxobolus pavlovskii (Myxozoa: Myxobolidae) includes an echinactinomyxon-type actinospore

Echinactinomyxon-type actinospores were found in a mixed-species oligochaete culture originating from the Temperate Water Fish Hatchery near Budapest, Hungary. On the basis of DNA sequence analysis, the actinospores were identified as Myxobolus pavlovskii (Akhmerov, 1954), the 18S rDNA sequence from myxospores of which is available in GenBank. Silver carp Hypophthalmichthys molitrix (Valenciennes) fry specimens were successfully infected by cohabitation with the echinactinomyxon-releasing oligochaetes, which confirmed the molecular data congruence. The echinactinomyxons and the myxospores that developed in the gills of exposed fish fry were analysed morphologically and on DNA basis. The infected gill tissue was examined histologically. As typical characters of M. pavlovskii, numerous small plasmodia were observed in the epithelia of gill lamellae. Plasmodia contained thousands of myxospores with polar capsules unequal in size and witl; large intercapsular processes. The 18S rDNA sequence from actinospores and those from myxospores originating from the experimentally infected fish were identical. The oligochaete species releasing actinospores was morphologically determined as Limnodrilus sp. This is the first record of an echinactinomyxon as an alternate stage within the genus Myxobolus.

Some quantitative data on bacterial attachment to pyrite

Bacterial attachment and biofilm formation are important for microbial bioleaching of metal sulfides, however, many details of the role played by bacteria are still unknown. Attachment as the first step in biofilm formation is critical. Our investigations are focused on these processes to control bacterial bioleaching processes. In moderately thermophilic pure cultures of Acidithiobacillus caldus and Leptospirillum ferriphilum, only the latter one showed significant attachment and biofilm formation to metal sulfides. In pure cultures of the mesophilic strains Acidithiobacillus spp., Leptospirillum ferrooxidans, Ferrimicrobium acidiphilum and a novel undescribed leaching organism designated SPIII/3, attachment was significant by L. ferrooxidans, At. thiooxidans and SPIII/3. In general, interaction of different species in mixed cultures resulted in increased attachment, increased production of extracellular polymeric substances (EPS) and leaching. Bacteria like Leptospirillum spp. enhanced the attachment of other species in mixed biofilms. Another finding is that large areas of the surface of minerals remain uncolonized, whereas at some places bacteria attach in clusters. Based on these findings we conclude that attachment and thereby bacterial leaching can be improved by the production of EPS by several strains especially in combination with Leptospirillum spp.

DETECTION OF PLANT-ASSOCIATED ERWINIA AND PANTOEA SPECIES BY MALDI-TOF MASS SPECTROSCOPY AND WITH NOVEL PCR PRIMERS

For fast and cost-efficient identification of bacteria we tested the Biotyper system (Bruker Daltonics) to detect Erwinia amylovora and other plant-associated bacteria by protein profiles from whole cells. Strains could also be identified by direct transfer of cells to the sample target, and the signals were compared to preceding protein extraction with formic acid. The identification scores obtained for extracted proteins were higher and more reliable than those of directly applied agar cultured cells. For verification of MALDI-TOF identification and for detection of several Erwinia species in mixed cultures we applied PCR primer sets designed from specific chromosomal sequence regions such as the gene cluster for biosynthesis of capsular EPS or the nucleotide sequences flanked by genes pstS and glmS. The novel primers readily detected E. amylovora fruit tree and raspberry strains, E. pyrifoliae, E. billingiae and E. tasmaniensis. The identification obtained by PCR agreed with the MALDI-TOF MS results. While PCR analysis is also applicable for detection of bacteria in mixed cultures obtained from plant samples, MALDI-TOF analysis identifies essentially cells from single colonies applied from agar plates or cultured in liquid medium. Both methods were combined for identification of bacteria in field samples and from artificially inoculated plant material.

Phytoremediation efficiency of a PAH-contaminated industrial soil using ryegrass, white clover, and celery as mono- and mixed cultures

Purpose Phytoremediation has been recognized as a promising technology for the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soils. However, little is known about how plant species and cropping patterns affect the process of phytoremediation removing PAHs. Therefore, the aim of this study was to investigate further the effects of monocultures or mixed cultures of different plant species on PAH phytoremediation.

Effect of phosphorus fluctuation caused by river water dilution in eutrophic lake on competition between blue-green alga Microcystis aeruginosa and diatom Cyclotella sp.

Tega-numa (Lake Tega) is one of the eutrophic lakes in Japan. For the improvement of water quality in Lake Tega, the North-chiba Water Conveyance Channel was constructed in 2000, which transfer water from Tone River into the lake. After 2000, the dominant species of diatoms, mainly Cyclotella sp., have been replacing blue-green algae, mainly Microcystis aeruginosa in Lake Tega. This transition of dominant species would be due to the dilution, but the detail mechanism has not been understood yet. This study examined the relationship between phosphorus fluctuation caused by river water dilution to Lake Tega and dominance of algal species, M. aeruginosa or Cyclotella sp. based on the single-species and the mixed-species culture experiments. The single-species culture experiment showed that the half-saturation constant and uptake rate of phosphorus were one order lower and seven times higher for M. aeruginosa than those for Cyclotella sp. These findings implied that M. aeruginosa would possess a potential for the growth and survival over Cyclotella sp. in the phosphorus limited condition. The superiority of M. aeruginosa was reflected in the outcome of the mixed-species culture experiment, i.e., dominance of M. aeruginosa, even phosphorus concentration was lowered to 0.01 mg-P/L. Therefore, it could be concluded that the decrease in phosphorus concentration due to the river water dilution to Lake Tega would be interpreted as a minor factor for the transition of dominant species from M. aeruginosa to Cyclotella sp.

Setup of an In Vitro Test System for Basic Studies on Biofilm Behavior of Mixed-Species Cultures with Dental and Periodontal Pathogens

BackgroundCaries and periodontitis are important human diseases associated with formation of multi-species biofilms. The involved bacteria are intensively studied to understand the molecular basis of the interactions in such biofilms. This study established a basic in vitro single and mixed-species culture model for oral bacteria combining three complimentary methods. The setup allows a rapid screening for effects in the mutual species interaction. Furthermore, it is easy to handle, inexpensive, and reproducible.Methods Streptococcus mitis, S. salivarius and S. sanguinis, typical inhabitants of the healthy oral cavity, S. mutans as main carriogenic species, and Porphyromonas gingivalis, Fusobacterium nucleatum, Parvimonas micra, S. intermedius and Aggregatibacter actinomycetemcomitans as periodontitis-associated bacteria, were investigated for their biofilm forming ability. Different liquid growth media were evaluated. Safranin-staining allowed monitoring of biofilm formation under the chosen conditions. Viable counts and microscopy permitted investigation of biofilm behavior in mixed-species and transwell setups.Findings S. mitis, F. nucleatum, P. gingivalis and P. micra failed to form biofilm structures. S. mutans, S. sanguinis, S. intermedius and S. salivarius established abundant biofilm masses in CDM/sucrose. A. actinomycetemcomitans formed patchy monolayers. For in depth analysis S. mitis, S. mutans and A. actinomycetemcomitans were chosen, because i) they are representatives of the physiological-, cariogenic and periodontitis-associated bacterial flora, respectively and ii) their difference in their biofilm forming ability. Microscopic analysis confirmed the results of safranin staining. Investigation of two species combinations of S. mitis with either S. mutans or A. actinomycetemcomitans revealed bacterial interactions influencing biofilm mass, biofilm structure and cell viability.ConclusionsThis setup shows safranin staining, microscopic analysis and viable counts together are crucial for basic examination and evaluation of biofilms. Our experiment generated meaningful results, exemplified by the noted S. mitis influence, and allows a fast decision about the most important bacterial interactions which should be investigated in depth.

Marine microalgae flocculation and focused beam reflectance measurement

The production of biodiesel from fast growing and lipid containing marine microalgal species is sustainably and economically more promising than fresh water microalgae. However, like fresh water microalgae, the process is limited by numerous factors such as the generally dilute nature of microalgal cultures, and the small size of microalgae cells, except for multi-cellular species such as Spirulina. Current dewatering technologies are sufficiently able to separate microalgae from the culture media; however the economics of the process makes these technologies ineffective as they are all too expensive when compared with the low cost of biodiesel from other sources. Centrifugation, which is one of the current technologies, is highly energy intensive especially in a large-scale setting. Filtration techniques such as tangential flow filtration has the potential to be a low cost dewatering technique, however there is a cost issue associated with the replacement of clogged membranes. Flocculation is another commonly used dewatering technique that has the advantage of using less energy under optimum conditions. Thus process development for marine microalgae flocculation could be an essential step to revolutionize biodiesel production from microalgae. In this work, mixed cultures of marine microalgal species were obtained from semi-continuous laboratory reactors and flocculation was investigated using polyelectrolyte (polymer) flocculants. Cationic, anionic and non-ionic polyelectrolyte flocculants were tested using the standard jar stirrer test at varying pH and temperature. All three flocculant types displayed suitability for microalgae flocculation with the cationic polymer obtaining the highest flocculation efficiency of 89.9% at an optimum concentration of 4 mg/L. Focused beam reflectance measurements (FBRM) showed real time changes in microalgal flocs size during the flocculation process. This data is essential to understand the kinetics of microalgal flocs formation, to ensure the stability of the floc formation process, and to monitor and evaluate the performance of the flocculation process.

Seasonal Dynamics of Diplectanum grouperi Parasitism on Wild Versus Cultured Groupers, Epinephelus spp., and the Linkage Between Infestation and Host Species Phylogeny

In this article, we examined the influences of the polycultured potential hosts on monogenean seasonality and the possible linkage between the infestations on different species of hosts and host phylogeny. The seasonality of Diplectanum grouperi , the dominant species, on wild versus cultured groupers, Epinephelus spp., was analyzed in Daya Bay, South China Sea, between April 2008 and January 2009. The prevalence, mean intensity, abundance, and variance/mean ratio were calculated for each species of host under polycultured and wild conditions. Except for the overall prevalence, which was slightly higher in autumn than that in summer under wild conditions, the mean intensity and variance were highest in summer, decreasing slightly in autumn to lowest levels in winter or spring. The infection level (prevalence, mean intensity, and abundance) was correlated with changes in water temperature during the sampling period, with a peak in summer, with the exception of E. awoara in autumn under wild conditions. The prevalence, intensity, and mean intensity of D. grouperi on Epinephelus spp. in the wild were much lower than those in experimental (mixed species) culture ponds. The correlation between the molecular phylogeny of 5 species of Epinephelus and the dendrogram based on the susceptibility to D. grouperi was not significant, which infers that variable susceptibilities of these Epinephelus species cannot be revealed by phylogenetic relationships determined from mitochondrial 16S rDNA and Cyt b gene sequences.

The degree of microbiome complexity influences the epithelial response to infection

BackgroundThe human microflora is known to be extremely complex, yet most pathogenesis research is conducted in mono-species models of infection. Consequently, it remains unclear whether the level of complexity of a host's indigenous flora can affect the virulence potential of pathogenic species. Furthermore, it remains unclear whether the colonization by commensal species affects a host cell's response to pathogenic species beyond the direct physical saturation of surface receptors, the sequestration of nutrients, the modulation of the physico-chemical environment in the oral cavity, or the production of bacteriocins. Using oral epithelial cells as a model, we hypothesized that the virulence of pathogenic species may vary depending on the complexity of the flora that interacts with host cells.ResultsThis is the first report that determines the global epithelial transcriptional response to co-culture with defined complex microbiota. In our model, human immortalized gingival keratinocytes (HIGK) were infected with mono- and mixed cultures of commensal and pathogenic species. The global transcriptional response of infected cells was validated and confirmed phenotypically. In our model, commensal species were able to modulate the expression of host genes with a broad diversity of physiological functions and antagonize the effect of pathogenic species at the cellular level. Unexpectedly, the inhibitory effect of commensal species was not correlated with its ability to inhibit adhesion or invasion by pathogenic species.ConclusionStudying the global transcriptome of epithelial cells to single and complex microbial challenges offers clues towards a better understanding of how bacteria-bacteria interactions and bacteria-host interactions impact the overall host response. This work provides evidence that the degree of complexity of a mixed microbiota does influence the transcriptional response to infection of host epithelial cells, and challenges the current dogma regarding the potential versus the actual pathogenicity of bacterial species. These findings support the concept that members of the commensal oral flora have evolved cellular mechanisms that directly modulate the host cell's response to pathogenic species and dampen their relative pathogenicity.